Http://cs1010/ UNIT 16 Characters and Strings

http://www.comp.nus.edu.sg/~cs1010/

UNIT 16

Characters and Strings

http://www.comp.nus.edu.sg/~cs1010

Unit 16: Characters and Strings

CS1010 (AY2014/5 Semester 1) Unit16 - 2© NUS

Objectives: Declare and manipulate data of char data type Learn fundamental operations on strings Write string processing programs

References: Lesson 1.4.1 Characters and Symbols Chapter 7: Strings and Pointers

Unit 16: Characters and Strings (1/2)


1. Motivation

2. Characters2.1 ASCII Table

2.2 Demo #1: Using Characters

2.3 Demo #2: Character I/O

2.4 Demo #3: Character Functions

2.5 Common Error

3. Strings3.1 Basics

3.2 String I/O

3.3 Demo #4: String I/O

3.4 Demo #5: Remove Vowels

3.5 Demo #6: Character Array without terminating ‘\0’

Unit 16: Characters and Strings (2/2)


4. String Functions

5. Pointer to String

6. Array of Strings

7. Demo #7: Using String Functions

8. Strings and Pointers

1. Motivation


Why study characters and strings? Hangman game – Player tries to guess a word by filling

in the blanks. Each incorrect guess brings the player closer to being “hanged”

Let’s play! http://www.hangman.no/

http://www.hangman.no/

2. Characters


In C, single characters are represented using the data type char

Character constants are written as symbols enclosed in single quotes Examples: 'g', '8', '*', ' ', '\n', '\0' Recall: Week 3 Exercise #7 NRIC Check Code

Characters are stored in one byte, and are encoded as numbers using the ASCII scheme.

ASCII (American Standard Code for Information Interchange), is one of the document coding schemes widely used today.

Unicode is another commonly used standard for multi-language texts.

2.1 Characters: ASCII Table


©The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

For example, character 'O' is 79 (row value 70 + col value 9 = 79).

For example, character 'O' is 79 (row value 70 + col value 9 = 79).

O

9

70

2.2 Demo #1: Using Characters (1/2)


// Unit16_CharacterDemo1.c#include <stdio.h>

int main(void) {

char grade = 'A', newgrade, ch;int value;

printf("grade = %c\n", grade);newgrade = grade + 2; printf("newgrade = %c\n", newgrade);printf("newgrade = %d\n", newgrade);

value = 65;printf("value = %d\n", value);printf("value = %c\n", value);

Unit16_CharacterDemo1.c

Declaring and initialising char variables.

Relationship between character and integer.

Using %cgrade = Anewgrade = C newgrade = 67

value = 65value = A

2.2 Demo #1: Using Characters (2/2)


if ('A' < 'c')printf("'A' is less than 'c'\n");

elseprintf("'A' is not less than 'c'\n");

for (ch = 'p'; ch <= 't'; ch++)printf("ch = %c\n", ch);

return 0;}

Comparing characters.

'A' is less than 'c'

ch = pch = qch = rch = sch = t

Using character variable as a loop variable.

ASCII value of 'A' is 65. ASCII value of 'c' is 99.

2.3 Demo #2: Character I/O


Besides scanf() and printf(), we can also use getchar() and putchar(). Note how they are used below:


int main(void) {char ch;

printf("Enter a character: ");ch = getchar();

printf("The character entered is ");putchar(ch);putchar('\n');

return 0;}


int main(void) {char ch;

printf("Enter a character: ");ch = getchar();

printf("The character entered is ");putchar(ch);putchar('\n');

return 0;}


Read a character from stdin.

Enter a character: WCharacter entered is W

Print a character to stdout.

2.4 Demo #3: Character Functions


Must include <ctype.h> to use these functions.

// Unit16_CharacterDemo3.c#include <stdio.h>#include <ctype.h>int main(void) {

char ch;

printf("Enter a character: ");ch = getchar();if (isalpha(ch)) {

if (isupper(ch)) {printf("'%c' is a uppercase-letter.\n", ch);printf("Converted to lowercase: %c\n", tolower(ch));

}if (islower(ch)) {

printf("'%c' is a lowercase-letter.\n", ch);printf("Converted to uppercase: %c\n", toupper(ch));

}}if (isdigit(ch)) printf("'%c' is a digit character.\n", ch);if (isalnum(ch)) printf("'%c' is an alphanumeric character.\n", ch);if (isspace(ch)) printf("'%c' is a whitespace character.\n", ch);if (ispunct(ch)) printf("'%c' is a punctuation character.\n", ch);return 0;

}


Download this program and test it out.For a complete list of character functions, refer to the Internet (eg: http://www.csd.uwo.ca/staff/magi/175/refs/char-funcs.html)

Note that tolower(ch) and toupper(ch) do NOT change ch!

http://www.csd.uwo.ca/staff/magi/175/refs/char-funcs.html

http://www.csd.uwo.ca/staff/magi/175/refs/char-funcs.html

2.5 Characters: Common Error


A character variable named z does not means it is equivalent to 'z' or it contains 'z'!

if (marks >= 80)return 'A';

else if (marks >= 70)return 'B';

else if (marks >= 60)return 'C';

. . .

if (marks >= 80)return 'A';

else if (marks >= 70)return 'B';

else if (marks >= 60)return 'C';

. . .

char A, B, C, D, F;

if (marks >= 80)return A;

else if (marks >= 70)return B;

else if (marks >= 60)return C;

. . .

char A, B, C, D, F;

if (marks >= 80)return A;

else if (marks >= 70)return B;

else if (marks >= 60)return C;

. . .char grade;if (marks >= 80)

grade = 'A';else if (marks >= 70)

grade = 'B';else if (marks >= 60)

grade = 'C';. . .return grade;

char grade;if (marks >= 80)

grade = 'A';else if (marks >= 70)

grade = 'B';else if (marks >= 60)

grade = 'C';. . .return grade;

3. Strings


We have seen arrays of numeric values (types int, float, double)

We have seen string constants printf("Average = %.2f", avg); #define ERROR "*****Error –"

A string is an array of characters, terminated by a null character '\0' (which has ASCII value of zero)

c s 1 0 1 0 \0

3.1 Strings: Basics


Declaration an array of characters• char str[6];

Assigning character to an element of an array of characters

• str[0] = 'e';• str[1] = 'g';• str[2] = 'g';• str[3] = '\0';

Initializer for string Two ways:

• char fruit_name[] = "apple";• char fruit_name[] = {'a','p','p','l','e','\0'};

e g g \0 ? ?

Without ‘\0’, it is just an array of character, not a string.

Do not need ‘\0’ as it is automatically added.

3.2 Strings: I/O (1/2)


Read string from stdin• fgets(str, size, stdin) // reads size – 1 char, • // or until newline• scanf("%s", str); // reads until white space

Print string to stdoutputs(str); // terminates with newlineprintf("%s\n", str);

Note: There is another function gets(str) to read a string interactively. However, due to security reason, we avoid it and use fgets() function instead.

3.2 Strings: I/O (2/2)


fgets() On interactive input, fgets() also reads in the newline

character

Hence, we may need to replace it with '\0' if necessary

fgets(str, size, stdin);len = strlen(str);if (str[len – 1] == '\n')

str[len – 1] = '\0';

e a t \n \0 ? ?User input: eat

3.3 Demo #4: String I/O


#include <stdio.h>#define LENGTH 10

int main(void) {char str[LENGTH];

printf("Enter string (at most %d characters): ", LENGTH-1);scanf("%s", str); printf("str = %s\n", str); return 0;

}



printf("Enter string (at most %d characters): ", LENGTH-1);scanf("%s", str); printf("str = %s\n", str); return 0;

}

Unit16_StringIO1.c



printf("Enter string (at most %d characters): ", LENGTH-1);fgets(str, LENGTH, stdin);printf("str = ");puts(str);return 0;

}



printf("Enter string (at most %d characters): ", LENGTH-1);fgets(str, LENGTH, stdin);printf("str = ");puts(str);return 0;

}

Unit16_StringIO2.c

Test out the programs with this input: My book

Output:str = My

Output:str = My book

Note that puts(str) adds a newline automatically.

3.4 Demo #5: Remove Vowels (1/2)


Write a program Unit16_RemoveVowels.c to remove all vowels in a given input string.

Assume the input string has at most 100 characters.

Sample run:

Enter a string: How HAVE you been, James?Changed string: Hw HV y bn, Jms?

3.4 Demo #5: Remove Vowels (2/2)


#include <stdio.h>#include <string.h>#include <ctype.h>int main(void) {

int i, len, count = 0;char str[101], newstr[101];

printf("Enter a string (at most 100 characters): ");fgets(str, 101, stdin); //what happens if you use scanf() here?len = strlen(str); // strlen() returns number of char in stringif (str[len – 1] == '\n')

str[len – 1] = '\0';len = strlen(str); // check length again

for (i=0; i<len; i++) {switch (toupper(str[i])) {

case 'A': case 'E':case 'I': case 'O': case 'U': break; default: newstr[count++] = str[i];

}}

newstr[count] = '\0';printf("New string: %s\n", newstr);return 0;

}

#include <stdio.h>#include <string.h>#include <ctype.h>int main(void) {

int i, len, count = 0;char str[101], newstr[101];

printf("Enter a string (at most 100 characters): ");fgets(str, 101, stdin); //what happens if you use scanf() here?len = strlen(str); // strlen() returns number of char in stringif (str[len – 1] == '\n')

str[len – 1] = '\0';len = strlen(str); // check length again

for (i=0; i<len; i++) {switch (toupper(str[i])) {

case 'A': case 'E':case 'I': case 'O': case 'U': break; default: newstr[count++] = str[i];

}}

newstr[count] = '\0';printf("New string: %s\n", newstr);return 0;

}

Unit16_RemoveVowels.c

Need to include <string.h> to use string functions such as strlen().

3.5 Demo #6: Character Array without terminating ‘\0’


What is the output of this code?

#include <stdio.h>#include <string.h>

int main(void) {char str[10];

str[0] = 'a';str[1] = 'p';str[2] = 'p';str[3] = 'l';str[4] = 'e';

printf("Length = %d\n", strlen(str));printf("str = %s\n", str);

return 0;}

Unit16_without_null_char.c

One possible output:Length = 8str = apple¿ø<

Compare the output if you add:str[5] = '\0';

or, you have:char str[10] = "apple";

%s and string functions work only on “true” strings. Without the terminating null character ‘\0’, string functions will not work properly.

printf() will print %s from the starting address of str until it encounters the ‘\0’ character.

4. String Functions (1/3)


C provides a library of string functions Must include <string.h> Table 7.3 (pg 509 – 514) http://www.edcc.edu/faculty/paul.bladek/c_string_functions.htm http://www.cs.cf.ac.uk/Dave/C/node19.html and other links you can find on the Internet

strcmp(s1, s2) Compare the ASCII values of the corresponding characters in

strings s1 and s2. Return

a negative integer if s1 is lexicographically less than s2, or a positive integer if s1 is lexicographically greater than s2, or 0 if s1 and s2 are equal.

strncmp(s1, s2, n) Compare first n characters of s1 and s2.

http://www.edcc.edu/faculty/paul.bladek/c_string_functions.htm

http://www.cs.cf.ac.uk/Dave/C/node19.html



strcpy(s1, s2) Copy the string pointed to by s2 into array pointed to by s1. Function returns s1. Example:

char name[10];strcpy(name, "Matthew");

The following assignment statement does not work:name = "Matthew";

What happens when string to be copied is too long? strcpy(name, "A very long name");

strncpy(s1, s2, n) Copy first n characters of string pointed to by s2 to s1.

M a t t h e w \0 ? ?

A v e r y l o n g n a m e \0



strstr(s1, s2) Returns a pointer to the first instance of string s2 in s1. Returns a NULL pointer if s2 is not found in s1,

We will use the functions above in Demo #7.

Read up on the above functions (Table 7.3 [pg 405 – 411] and Table 7.4 [pg 412 – 413]) We will do some more exercises on them next week

Other functions (atoi, strcat, strchr, strtok, etc.) We will explore these in your discussion session

5. Pointer to String (1/2)


#include <stdio.h>#include <string.h>int main(void) {

char name[12] = "Chan Tan"; char *namePtr = "Chan Tan";

printf("name = %s\n", name);printf("namePtr = %s\n", namePtr);printf("Address of 1st array element for name = %p\n", name);printf("Address of 1st array element for namePtr = %p\n",namePtr);

strcpy(name, "Lee Hsu");namePtr = "Lee Hsu";

printf("name = %s\n", name);printf("namePtr = %s\n", namePtr);printf("Address of 1st array element for name = %p\n", name);printf("Address of 1st array element for namePtr = %p\n",namePtr);

}

Unit16_StringPointer.c

name is a character array of 12 elements. namePtr is a pointer to a character. Both have strings assigned. Difference is name sets aside space for 12 characters, but namePtr is a char pointer variable that is initialized to point to a string constant of 9 characters.

name updated using strcpy(). namePtr assigned to another string using =.

Address of first array element for name remains constant, string assigned to namePtr changes on new assignment.

5. Pointer to String (2/2)


Comparison

char name[12] = "Chan Tan";

char *namePtr = "Chan Tan";namePtr

C h a n T a n \0

C h a n T a n \0 \0 \0\0name[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

6. Array of Strings


Declarationchar fruits[MAXNUM][STRSIZE]; // where MAXNUM is the maximum number of

names// and STRSIZE is the size of each name

Initializationchar fruits[][6] = {"apple", "mango",

"pear"};or

char fruits[3][6] = {"apple", "mango", "pear"};

Outputprintf("fruits: %s %s\n", fruits[0], fruits[1]);printf("character: %c\n", fruits[2][1]);

fruits: apple mangocharacter: e

7. Demo #7: Using String Functions


#include <stdio.h>#include <string.h>#define MAX_LEN 10

int main(void) {char s1[MAX_LEN + 1], s2[MAX_LEN + 1], *p;int len;

printf("Enter string (at most %d characters) for s1: ", MAX_LEN);fgets(s1, MAX_LEN+1, stdin);len = strlen(s1);if (s1[len – 1] == '\n') s1[len – 1] = '\0';


printf("strcmp(s1,s2) = %d\n", strcmp(s1,s2));

p = strstr(s1,s2);if (p != NULL) printf("strstr(s1,s2) returns %s\n", p);else printf("strstr(s1,s2) returns NULL\n");

strcpy(s1,s2);printf("After strcpy(s1,s2), s1 = %s\n", s1);return 0;

}

#include <stdio.h>#include <string.h>#define MAX_LEN 10

int main(void) {char s1[MAX_LEN + 1], s2[MAX_LEN + 1], *p;int len;



printf("strcmp(s1,s2) = %d\n", strcmp(s1,s2));

p = strstr(s1,s2);if (p != NULL) printf("strstr(s1,s2) returns %s\n", p);else printf("strstr(s1,s2) returns NULL\n");

strcpy(s1,s2);printf("After strcpy(s1,s2), s1 = %s\n", s1);return 0;

}

Unit16_StringFunctions.c

8. Strings and Pointers (1/4)


We discussed in Unit #9 Section 4 that an array name is a pointer (that points to the first array element)

Likewise, since a string is physically an array of characters, the name of a string is also a pointer (that points to the first character of the string)

char str[] = "apple";

printf("1st character: %c\n", str[0]);printf("1st character: %c\n", *str);

printf("5th character: %c\n", str[4]);printf("5th character: %c\n", *(str+4));

Unit16_String_vs_Pointer.c

1st character: a1st character: a5th character: e5th character: e



Unit16_strlen.c shows how we could compute the length of a string if we are not using strlen()

See full program on CS1010 website

int mystrlen(char *p) {int count = 0;

while (*p != '\0') {count++;p++;

}

return count;}

Unit16_strlen.c



Since ASCII value of null character '\0' is zero, the condition in the while loop is equivalent to (*p != 0) and that can be further simplified to just (*p) (see left box)

We can combine *p with p++ (see right box) (why?)


while (*p) {count++;p++;

}

return count;}


while (*p++) {count++;

}

return count;}

Unit16_strlen_v2.c



How to interpret the following?

while (*p++)

Check whether *p is 0 (that is, whether *p is the null character ‘\0’)…

Then, increment p by 1 (so that p points to the next character).Not increment *p by 1!

(*p++) is not the same as (*p)++(*p)++ is to increment *p (the character that p points to) by 1. (Hence, if p is pointing to character ‘a’, that character becomes ‘b’.)

Extra topics


2 additional topics that are not in the syllabus are in the file Unit16_Extra.pptx Array of Pointers to Strings

Command-line arguments

Summary


In this unit, you have learned about Characters

Declaring and using characters Characters I/O Character functions

Strings Declaring and initialising strings String I/O String functions Array of strings

End of File


Documents

Http://cs1010/ UNIT 16 Characters and Strings